The way in which reagents are mixed can affect significantly the size distribution and morphology of the solid product. The process of precipitation can be so fast that even the micro-mixing process that is usually the fastest of the turbulent mixing sequence can affect the course of precipitation and the quality of precipitation product. Effects of mixing are particularly high in the case of the double-feed precipitation process. The process of BaSO4 precipitation in a double-feed semibatch stirred tank reactor is modelled using a non-equilibrium multiple-time-scale mixing model and a beta distribution of the mixture fraction in combination with a simple conditional moment closure based on linear interpolation of local instantaneous reactant concentration values. Mixing, nucleation and crystal growth are modeled in the feed zone using CFD, whereas nucleation and growth of crystals in the bulk are simulated using a simple concept of the semibatch well-mixed tank. Modelling predicts all trends observed in experiments.